Device for venting gases from a tank

ABSTRACT

A device for venting gases from a cargo tank of a tanker ship when being filled with cargo, including a housing, an annular diaphragm of resilient material secured at its outer periphery to the housing and a spherical seat so positioned as to close the aperture in the diaphragm and deform the diaphragm to generally frusto-conical shape whereby the diaphragm is in forced engagement with the seat and a pressure is required in the tank to lift the diaphragm off the seat to vent gases. The diaphragm can pass over the equator of the seat when a negative pressure is created upon unloading so as to allow air into the tanks. The position of the seat member is adjustable in order to select the pressure at which the device starts to vent gases. Gases vented are formed by the device into a jet which is directed upwardly from the deck of the tanker ship.

United States Patent Lowe [ 1 Aug. 1,1972

. '22 Filed:

[54] DEVICE FOR VENTING GASES FROM A TANK [72] Inventor: Desmond Lowe, Windlesham, En-

gland [73] Assignee: National Research Development 1 Corporation, London, England Nov. 20, 1970 21 Appl. No.: 91,346

[30] Foreign Application Priority Data Primary Examiner-M. Cary Nelson Assistant Examiner-Robert J. Miller Attorney-Cushman, Darby and Cushman [s7] ABSTRACT A device for venting gases from a cargo tank of a tanker ship when being filled with cargo, including a housing, an annular diaphragm of resilient material secured at its outer periphery to the housing and a spherical seat so positioned as to close-the aperture in the diaphragm and deform the diaphragm to generally frusto-conical shape whereby the diaphragm is in forced engagement with the seat and a pressure is required in the tank to lift the diaphragm off the seat to vent gases. The diaphragm can pass over the equator of the seat when anegative pressure is created upon unloading so as to allow air into the tanks. The position of the seat member is adjustable in order to select the pressure at which the device starts to vent gases. Gases vented are formed by the device into a jet which is directed upwardly from the deck of the tanker ship.

15 Claims, 8 Drawing Figures PATENTEDAUB 1 m2 SHEET 3 [IF 6 PATENTEUMM; 1 m2 SHEET 6 BF 6 ll SQEQQ QNQ DEVICE FOR VENTING GASES FROM A TANK This invention relates to devices for venting gases from a tank being filled with liquid at a variable rate and for forming the gases into a jet.

It is well known that gases in cargo tanks of crude oil or petroleum product tanker ships are combustible and may be in a combustible mixture with air. When cargo is introduced into the tanks the gases in the tanks have to be evacuated from the tanks. It would not be satisfactory to release them in such a manner, as through a hatch in the deck, that they emerge immediately above the deck with only a very small velocity, because they would be, and might remain, accessible to any source of ignition on, or in the region of, the deck. 7

Proposals have been made for leading the gases from the individual tanks along ducts to a stack extending upwardly to a height above the deck and from the top of which the gases are discharged so as to disperse However, such proposals for devices producing jets have involved the use of hinged or otherwise movable metal components which are liable to corrosion and may stick in an undesired attitude thereby lowering the velocity of the gases so that they are not ejected to a safe height above the deck. If such devices remain open when loading has ceased then there is a path for flames to the interior of the tank.

It is an object of the present invention to provide a device for causing gases displaced from the cargo tank of a tanker ship to be directed to a safe region above the deck of the ship without the need to provide ducting to such safe region and which'device overcomes the disadvantages of known devices for this purpose.

According to the present invention there is provided a device for venting gases from a tank being filled with liquid at a variable rate and for forming the gases into a jet, said device including a housing having first and second openings at opposite sides, an annular diaphragm of resilient material disposed in the housing between the openings and having its outer periphery secured to the housing in a plane, a seat member having a surface conforming to a solid of revolution with its axis normal to said plane and substantially at the center of the annular diaphragm and being there supported by a mounting, the seat member and mounting permitting free access of gas to and from the diaphragm and the mounting permitting free movement of the diaphragm,

circular aperture in the diaphragm when the diaphragm area of the aperture in the diaphragm when unstressed and the positional relationship of the equator of the seat member and the outer periphery of the diaphragm being such that without a pressure differential on its surfaces the inner periphery of the diaphragm is in forced engagement with the seat member at one or the other side of the equator and the diaphragm is deformed from its planar condition and such that the inner periphery of the diaphragm can pass over the equator by virtue of a pressure differential on the surfaces of the diaphragm, the diaphragm being so formed that upon a predetermined positive pressure appearing at the first opening facing the tank the diaphragm moves off the seat member and the diaphragm becomes bowl-shaped having the concave surface facing the first opening.

Conveniently, the seat member is spherical.

Preferably the diaphragm is of decreasing thickness considered in the direction from its radially outer to its radially inner periphery, and the diaphragm may be approximately twice as thick at its outer periphery as at its inner periphery.

In one embodiment of the present invention the thickness of the diaphragm at its inner periphery is approximately 0.02 inch.

Preferably the position of the seat member is adjustable in the direction normal to the plane of the periphery of the diaphragm.

The housing may include two separable parts rigidly securable together to trap the outer peripheral portion of the diaphragm therebetween and one of said parts may have an annular groove for receiving an annular bead on one surface of the diaphragm.

The housing may include a tubular wall bounding a passage, the said first opening being at one end of said passage and the diaphragm being at the outer end of I said passage, said tubular wall having an outwardly directed flange at said one end for securing the device to the tank. The diaphragm and seat member are preferably, in certain embodiments,'formed of electrically-conductive material.

is in an unstressedcondition, the seat member being of reducingcross-sectional area in both axial directions away from the equator, the relationship of the crosssectional area of the seat member at its equator and the The present invention also resides in a diaphragm for use in a device in accordance with the invention, the diaphragm comprising an annularmember of resilient material adapted to be secured at its outer periphery to the housing of the device.

The annular member preferably is tapered radially being thicker at the outer periphery than at the inner periphery. The ratio of the thicknesses of the annular member at its radially outer and radially inner peripheries may be approximately 2:1.

When the diameter of the outer periphery is approximately 10 inches, the radially outer thickness may be approximately 0.040 inch and the radially inner thickness may be approximately 0.020 inch.

The annular member may have an annular bead, one surface thereof adjacent the radially outer periphery.

The present invention also resides in a tankership having a plurality of devices in accordance with the invention for venting gases from the cargo tanks thereof. I v An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

FIG. lis a diagrammatic illustration of portionsof a present invention;

FIG. 2 illustrates a cross-section in a vertical plane of a device in accordance with the present invention;

v FIG. 3 illustrates a perspective view of the diaphragm included in the device illustrated in FIG. 2;

- FIG. 4 illustrates a cross-sectional view of the diaphragm illustrated in FIG. 3;

FIG. 5 illustrates a scrap view of the seat member which might otherwise cause extrusion of the diaphragm from-between the flange and clamping ring.

A seat member 43 is mounted approximately in the end of the passage 19 adjacent the diaphragm 35 by and a portion of the diaphragm illustrated in FIG. 2 and on an enlarged scale;

FIG. 6 is similar to FIG. 2 but illustrating a difl'erent operative condition of the diaphragm;

FIG. 7 is a graph of jet velocity measured on and to .FIG. 8 is a graph of jet velocity measured on and to each side of the center line of the jet measured approximately nineteen feet above the device.

In FIG. 1 there are diagrammatically illustrated portions of an oil tanker ship which includes cargo tanks 1. The tanks 1 are bounded by bulkheads 3, the deck 5 and the bottom; 7. Oil cargo is deliveredto anddischarging of the tanks 1 may be controlled.

:When the tanks l are empty priorto loading they contain combustible gases. Devices are provided on the deck for venting gases displaced from the tanks during loading. One of such devices 15 is illustrated in 17 bounding a passage 19. At one end the wall 17 has an outwardly directed flange 21 apertured for cooperation with studs 23 for securing the device to the deck 5. The deck 5 has an aperture 25 aligned with the passage 19. At the end remote from the flange 21, the tubular wall'17 has a radially outwardly directed flange 27. The radially inner portion of the flange 27 is so shaped as to provide a generally frusto-conical surface 29 co-axial with the axis of the passage 19 and of reducing diameter in the direction inwardly of the passage 19. The flange 27 has formed in its radial surface 1 remote from the flange 21 an annular groove 31 for receiving an annular bead 33-on one radial surface of I an annular diaphragm 35. The groove 31 is spaced radially outwardsof the radially inner edge of the radial surface of the flange 27 so that a radially outer annular portion of the diaphragm 35 lies against a planarannular portion of the surface of the flange 27 andis trapped between the flange 27 and aclamping ring 37 which is secured to the flangeby bolts 39. The radially inner edge 41 of the clamping ring 37 is radiussed so as to present an arcuate surface rather than a sharp edge to the diaphragm. The. annular portion-38 of the radial surface of the flange 37 located radiallyoutside the each side of the center line of the jet for four different values of pressure measured approximately five inches above the plane of the periphery of the diaphragm; and

mounting means which include an axially directed rod carried by a spider 48 secured to the tubular wall 17. The rod 45 is threadedly engaged with the seat member 43 whereby the position of the seat member in the direction axially of the passage 19 may be adjusted. A lock nut 49 is provided to secure the seat member in its desired position.

In the embodiment of the invention illustrated and being described the seat member-43 is a sphere and its center is disposed on theline of theaxis of the passage 19 which axis also passes throughthe geometric center of the annular groove 33 is disposed.

The central aperture 47 in the diaphragm has a diameter, when the diaphragm is'in an unstressed and planar condition, less than the diameter of the spherical seat member 43 and the seat member 43 is so disposed along the line of the axis of the passage 19 that the diaphragm is deformed to an approximately frustoconical shape as illustrated in-FIG. 2 when there is no I pressure differential on the-opposite faces of y the diaphragm. In this way, the inner periphery. of the diaphragm is in forced contact with the seat member 43 and a predetermined positive pressure in the passage 19 is required to lift the diaphragm out of contact with the seat member 43. In the condition of no pressure dif- FIG; 2 and comprises'a housing including a tubular wall 470' dimensions and characteristics:

Diameter of passage 19 6 inches Diameter of seat member 43 H6 inches Outer diameter of diaphragm 35 10 inches Effective diameter of diaphragm 35 8% inches Radial dimension of bead 33 I50 inch Axial thickness of bead 33 0.12 inch Diameter of aperture 47 unstressed 1% inch Thickness of diaphragm immediately adjacent bead 33 0.04 inch Thickness of diaphragm at inner periphery, unstressed 0.02 inch In relation to the material of the diaphragm: Specific gravity 1.14 Hardness 41-50 BS 1 Tensile strength 1, 0 psi, minimum Elongation at break 650%mini r'num ferential on the surfaces of the diaphragm a radially inner annular portion of the diaphragm conforms to and is in contact with the seat member as may be seen in FIG. 4.

The diaphragm 35 is formed of resilient material and,

in the present example, is radially tapered, it being thicker at its outer periphery (disregarding the bead 33) than at its inner periphery.

As will'be realized, the housing comprising the tubular wall l7 and clamping ring 37' is open at opposite sides, i.e. at the end of the tubular wall remote from the clamping ring-and at the clampingring' a One device as described above has the following Volume increase in ATF 70/30 24 hours at 40 0.

- +30 maximum u groove 31 is steppedup so as to prevent excessive tightening of the clamping ring 37 on to the flange The devices 15 are secured to the deck 5 sealing 31 in which the diaphragm bead which case the tanks 1 contain only traces of oil and volatile combustible gases. When oil is introduced into a particular tank 1 through the ring main 9 and branch 11 pressure builds up in the tank 1 due to the influx of oil and the lack of an escape path for gas above the oil. At the beginning of loading the diaphragms 35 have a shape substantially as illustrated in FIG. 2. In this condition of the device no gas can escape to the atmosphere because the radially inner annular portion of the diaphragm is in surface-to-surfacecontact with the seat member 43 as illustrated in FIG. 4. Because the diaphragm is in forced contact with the seat member 43 by virtue of the stresses induced in the diaphragm by the displacement of the diaphragm from its natural planar condition, a positive pressure in the passage 19 is necessary to lift the diaphragm off the seat member to allow gas to escape through the aperture 47 in the diaphragm 35. The magnitude of thepositive pressure necessary to lift the diaphragm off the seat member is dependent upon the characteristics of the material of the diaphragm and upon the structure of the diaphragm and in the case of a particular diaphragm it is dependent upon the magnitude of the displacement of the diaphragm from its planar state by the seat member. Such displacement is dependent upon the relative sizes of the seat member and aperture 47 and upon the position of the equator of the seat member (i.e. the diametral plane normal to the axis of the passage 19) relative to the plane of the outer periphery of the diaphragm. It will therefore be realized that for a particular diaphragm. and a particular seat member the magnitude of the positive pressure required in the passage 19 to lift the diaphragm off the seat member is selected by selecting the position of the seat member axially of the passage 19.

As the gas pressure increases in the tank 1 upon the continued loading of oil into the tank 1 so the diaphragm deforms and takes on a curved shape with the concavity facing downwards whilst the inner portion of the diaphragm remains in contact with the seat member.

Eventually a positive pressure is arrived at in the tank 1 and passage 19 which causes the diaphragm to lift ofi the seat member and allow gas to escape through the aperture 47 in the diaphragm. Because the gas is at a super-atmospheric pressure and by virtue of its flow through the circular aperture 47 it forms a jet directed upwardly away from the deck 5.

The size of each device and the number of devices characterlstlcs of the device and the number of devlces the rate of passage of gas is such that the gas pressure in the tank does not increase. If the rate of loading of the tank varies then the deformation of the diaphragm and hence the area of the aperture will vary.

FIG. 5 illustrates the state of the diaphragm under stable conditions at normal full loading rate. It will be seen that the diaphragm is bowl-shaped with the concave surface of the diaphragm facing down the passage 19.

In one device as described above the following results were-achieved when the seat member was positioned with its equator located one-eighth inch above the plane of the periphery of the diaphragm.

H Q L r) V w 11.2 264 2.07 2.47 132 1,188 14.5 429 2.33 2.91 155 1,934 16.1 543 2.42 3.12 170 2,444 17.6 657 2.56 3.38 176 2,958 18.7 7 53 2.61 3.50 188 3,391 19.9 865 2.69 3.70 193 3,893 20.5 930 2.74 3.75 202 4,185 21.2 1,006 2.76 3.90 202 4,527

In the above table H Pressure in tank in inches WG Q= Plow through device c.f.m. at standard pressure L Height of aperture above periphery of diaphragm in inches D= Diameter of aperture in diaphragm in inches I Mean velocity of jet immediately above device in ft. per sec.

W= Mass of gas vented by the device in lbs per hour.

The device passes gas at pressures (H) below the lowest value in the table above. For example, at H 2.8 the following values were achieved It will be observed that even with a pressure in the tank as low as 2.8 inches WG the mean velocity of the jet produced is 48.2 ft./sec.

It is stressed that by appropriate selection of the used in any one tank in relation to the intended rate of 15 associated with each tank are so selected that at the normal full rate at which the tank is loaded with oil, the devices 15 allow the displaced gas to escape without increase in pressure in the tank.

However, the rate at which the device 15 allows gas to escape during the time immediately after the diaphragm has lifted off the seat member is less than .the rate at which oil is loaded into the tank so that pressure in the tank increases. Such increase in the gas loading of the tank, the transitional stages prior to stable venting conditions being achieved may be passed through quickly.

It will be realized that the minimum mean velocity of the jet produced by the jet during normal loading conditions may be selected so as to achieve recommended values. Suggestions have been made that the minimum means velocity should be ft./sec. at the jet producing device so that the gases are jetted to a height above the deck where they will be naturally dispersed and away from any source of ignition at deck level.

FIG. 7 shows the results of measurements of velocity with the velocity sensor being moved along a diameter of the jet between three inches to one side of the jet center line and 3 inches to the other side of the jet center line. FIG. 7 shows the results obtained for four different values of pressure (H) in the tank, the values being H= 18.66; 16.32; 10.02 and 5.82. At these pressures the flow rates (Q) were Q= 732; 573; 239.6 and respectively. The measurements of velocity were taken approximately 4 inches above the plane of the periphery of the diaphragm. It will be observed that the jet is well defined and that the velocity gradient at the boundary of the jet is sharp.

FIG. 8 shows the results of measurements of the velocity of the jet at a height approximately 19 ft.

, the tanks during normal loading conditions may be held below the maximumby the appropriate selection of the 7 size and number of the jet-producing devices bearing in mind that each device has a maximum throughput. It has been found thatthe diaphragm described above was not permanently distorted after it had been run with the diameter of the aperture at inches, i.e. only 1 inch less than the diameter of the passage 19.

It has been found that'raising the seat member serves I to raise the pressure at which the device starts to vent gas but that aftertransient pressures have been passed through, the position of the seat member has little or no effect upon venting conditions. The maximum height of the seat member is, in part, determined by the relationship of the equatorial size of the seat member and the size of i the aperture in the unstressed diaphragm.

Evidently the condition can be achieved in which the diaphragm is so stressed that the aperture has the same size as the seat at its equator. To avoid this condition seat members with cross-sectional areas at their equators larger than that (i.e. .2 inch diameter) described above may be employed providing that the seat member and the bounding wall of the gas passage to the diaphragm do not cause an undesired restriction to gas flow.

It is believed that the superior jet produced by a device in accordance with the present invention may be attributable at least in part to the bowl shape of the diaphragm when the device is venting and to gas disposed in the concavity of the bowl shape diaphragm which gas might be regarded as dead and providing for the gas moving towards the aperture in the diaphragm a desired nozzle shape.

It will be realized that the rate of loading of tanks .may vary and in particular the rate of loading is -reduced when the tanks are nearly full. Such a procedure is sometimes known as topping-up. During such reduced rate of loading the rate of displacement of gas is reduced. When this reduced rate of displacement of gas occurs the pressure on the underside of the diaphragm decreases and hence the deformation of the known to cause collapse of a tank when precautions to l I allow air into the tank had'not been taken.

diaphragm decreases and the area of the aperture in the diaphragm becomes less so that the rate of discharge of gas is decreased whereby the pressure in the tank does not continue falling and hence the velocity of the jet is also held abovethat low value which would have eventually occurred if the larger diameter of the aperture had been maintained. v I

' ltis an additional advantageof a device in accordance with thepresentinventionthat it allows air to enter the tank during unloading thereby preventing a vacuum. being createdwithin the tank which has been When oil is withdrawn from the tank 1 through the ring main 9, gas pressure above the oil within the tank is lowered below atmospheric. Such lowering of the gas pressure causes the diaphragmto deform from its condition illustrated in FIG. 2 and become downwardly convex. After continued pumping a pressure differential on the two surfaces of the diaphragm is created, which causes the 'inner periphery of the diaphragm to pass over the equator of the seat member and to allow air to enter the tank to prevent a further decrease inthe pressure within the tank. If pumping. I

ceases the diaphragm closes on to the seat member and when the tank is again loaded. the diaphragm passes upwardly over the equator of the seat member.

Whilst the diaphragm has been 'specificallydescribed above as being radially tapered, it has-been found that acceptable performance may be achieved with a diaphragm which is not tapered. However it has been found that a-higher mass flow of gas at a given velocity may .be achieved at a lower .tank diaphragm is tapered.

Whilst the seat member has 1 been specifically described above as being spherical it is to be understood that other shapes of seat member may be included in devices in accordance with the present invention. However, it is necessary that the seat member should be formed as a geometric solid of revolution and that it should have an equatorial region of maximum cross-sectional area and be of reducing cross-section in pressure if the both axial directions away from the equator. Evidently the equatorial cross-sectional area of the seat member should be greater than the area of the aperture in the diaphragm when'unstressed. I

A flame screen may be fitted inthe passage 19 but no form of obstruction which might diffuse orinterrupt the jet-of gas produced by the device should. be

disposed above the device during jetting although a flame screen may be disposed over the device when unloading. The lack of an obstruction above the clamping ring 37 allows easy inspection of the diaphragm during venting in order'to check that it is operating satisfactorily. For example, it may be possible tocheck that all the diaphragms have their invertedbowl shape or igloo shape from the bridge of the ship. a

' When a device in accordance with the present invention is employed for producing a jet of combustible gases, as in the case of gases in the cargo tanks of tanker ships, it is desirable that the diaphragm and seat member should be electrically conducting and that they should have electrical paths to-earth and in this case any risk involved from the creation of static electricityby the fast flowing gas is avoided. However, it is tive material which may be used for the'diaphragm of the device is a silicone rubber produced by Midland Silicones Limited under the reference 8M4.

An alternative manner of avoiding electrostatic charge build up is to include a low intensity radioactive source so as to ionise gases passing through the device.

Evidently when the device is to be used for venting the cargo tank of a tanker ship, the material of the diaphragm should be oil-resistant. It has been found ad-. vantageous if the ratio of the diameters of the outer and inner peripheries of the diaphragm is about 8:1. It has also been found advantageous if the ratio of the thickness of the diaphragm at its outer and inner peripheries (excluding any bead at the outer periphery) is about 2:1 and that the thickness at the inner periphery is in the range 0.01 inch to 0.125 inch.

1 claim:

1. A device for venting gases from a tank being filled with liquid at a variable rate and for forming the gases into a jet, said device including:

a housing;

said housing having first and second openings at opposite sides thereof;

an annular diaphragm formed of resilient material and disposed in said housing between said openings;

said diaphragm having an outer periphery secured to said housing in a plane;

a seat member;

said seat member having a surface conforming to a solid of revolution;

mounting means for mounting said seat member with its axis normal to said plane and substantially at the center of said diaphragm;

said seat member and said mounting means being adapted to permit free access of gas to and from said diaphragm;

said mounting means being adapted to permit free movement of said diaphragm;

said seat member having an equatorial region of maximum cross-sectional area greater than the area of the circular aperture in the diaphragm when the diaphragm is in an unstressed condition; said seat member being of reducing cross-sectional area in both axial directions away from its equator; the relationship of the cross-sectional area of said seat member at its equator and the area of the aperture in the diaphragm when unstressed and the positional relationship of the equator of said seat member and the plane of the outer periphery of the diaphragm being such that without a pressure differential on the surfaces of the diaphragm the inner periphery of the diaphragm is in forced engagement with said seat member at one or the other side of the equator and said diaphragm is deformed from its planar condition and. such that said first opening.

2. A device according to claim 1, wherein said seat member is spherical.

fiaif$$$iflii fiii i 'considered in the direction from its radially outer to its radi ally inner periphery.

4. A device according to claim 3, wherein:

said diaphragm is approximately twice as thick at its outer periphery as at its inner periphery.

5. A device according to claim 4, wherein:

the thickness of said diaphragm at its inner periphery is approximately 0.02 inch.

6. A device according to claim 1, including:

means for adjusting the position of said seat member in the direction normal to said plane. 1

7. A device according to claim 1, wherein:

said housing includes two separable parts adapted to be rigidly secured together to trap the outer peripheral portion of said diaphragm therebetween;

said diaphragm has on one of its surfaces an annular bead; and

one of said parts has an annular groove adapted to receive said bead.

8. A device according to claim 1, wherein:

said housing includes a tubular wall adapted to bound a passage;

the said first opening being at one end of said passage;

said diaphragm being at the other end of said passage;

said tubular wall having an outwardly directed flange at said first end adapted for securing the device to the tank.

9. A device according to claim 1, wherein:

said diaphragm and said seat member are formed of electrically conductive material.

10. A diaphragm adapted for use in a device according to claim 1, comprising:

an annular member of resilient material adapted to be secured at its outer periphery to the housing of said device.

1 l. A diaphragm according to claim 10, wherein: said annular member is tapered radially being thicker at its outer periphery than at its inner periphery.

12. A diaphragm according to claim 1 1 wherein:

the ratio of the thicknesses of the annular member at its radially outer and radially inner peripheries is approximately 2: l.

13. A diaphragm according to claim 12, wherein:

the diameter of the outer periphery is approximately 10 inches and the radially outer thickness is approximately 0.040 inch and the radially inner thickness is approximately 0.020 inch.-

14..A diaphragm according to claim 10, wherein:

the annular member has an annular bead on one surface thereof adjacent the radially outer periphery.

15. A diaphragm according to claim 10, wherein:

the annular member is electrically conductive. 

1. A device for venting gases from a tank being filled with liquid at a variable rate and for forming the gases into a jet, said device including: a housing; said housing having first and second openings at opposite sides thereof; an annular diaphragm formed of resilient material and disposed in said housing between said openings; said diaphragm having an outer periphery secured to said housing in a plane; a seat member; said seat member having a surface conforming to a solid of revolution; mounting means for mounting said seat member with its axis normal to said plane and substantially at the center of said diaphragm; said seat member and said mounting means being adapted to permit free access of gas to and from said diaphragm; said mounting means being adapted to permit free movement of said diaphragm; said seat member having an equatorial region of maximum crosssectional area greater than the area of the circular aperture in the diaphragm when the diaphragm is in an unstressed condition; said seat member being of reducing cross-sectional area in both axial directions away from its equator; the relationship of the cross-sectional area of said seat member at its equator and the area of the aperture in the diaphragm when unstressed and the positional relationship of the equator of said seat member and the plane of the outer periphery of the diaphragm being such that without a pressure differential on the surfaces of the diaphragm the inner periphery of the diaphragm is in forced engagement with said seat member at one or the other side of the equator and said diaphragm is deformed from its planar condition and such that the inner periphery of the diaphragm can pass over the equator by virtue of a pressure differential on the surfaces of the diaphragm; said diaphragm being adapted to move off said seat member and to assume a bowl-shape having the concave surface towards said first opening, which said first opening is adapted to face said tank upon a predetermined positive pressure appearing at said first opening.
 2. A device according to claim 1, wherein said seat member is spherical.
 3. A device according to claim 1, wherein: said diaphragm is of decreasing thickness considered in the direction from its radially outer to its radially inner periphery.
 4. A device according to claim 3, wherein: said diaphragm is approximately twice as thick at its outer periphery as at its inner periphery.
 5. A device according to claim 4, wherein: the thickness of said diaphragm at its inner periphery is approximately 0.02 inch.
 6. A device according to claim 1, including: means for adjusting the position of said seat member in the direction normal to said plane.
 7. A device according to claim 1, wherein: said housing includes two separable parts adapted to be rigidly secured together to trap the outer peripheral portion of said diaphragm therebetween; said diaphragm has on one of its surfaces an annular bead; and one of said parts has an annular groove adapted to receive said beAd.
 8. A device according to claim 1, wherein: said housing includes a tubular wall adapted to bound a passage; the said first opening being at one end of said passage; said diaphragm being at the other end of said passage; said tubular wall having an outwardly directed flange at said first end adapted for securing the device to the tank.
 9. A device according to claim 1, wherein: said diaphragm and said seat member are formed of electrically conductive material.
 10. A diaphragm adapted for use in a device according to claim 1, comprising: an annular member of resilient material adapted to be secured at its outer periphery to the housing of said device.
 11. A diaphragm according to claim 10, wherein: said annular member is tapered radially being thicker at its outer periphery than at its inner periphery.
 12. A diaphragm according to claim 11, wherein: the ratio of the thicknesses of the annular member at its radially outer and radially inner peripheries is approximately 2:1.
 13. A diaphragm according to claim 12, wherein: the diameter of the outer periphery is approximately 10 inches and the radially outer thickness is approximately 0.040 inch and the radially inner thickness is approximately 0.020 inch.
 14. A diaphragm according to claim 10, wherein: the annular member has an annular bead on one surface thereof adjacent the radially outer periphery.
 15. A diaphragm according to claim 10, wherein: the annular member is electrically conductive. 